This invention concerns a gas-generating substance mixture, which is useful to produce propellant gas for passenger protection devices in motor vehicles, in particular airbag systems.
Passive safety devices for motor vehicles, such as e.g. airbag systems, serve to protect vehicle passengers from injuries in the event of a vehicle collision occurring. To this end, a gas generator contains a gas-generating substance mixture inside a combustion chamber; this gas-generating substance mixture takes the form of tablets, pellets, or granules, and, on activation, produces a propellant gas which in turn blows up a gas bag; this will, for example, prevent vehicle passengers from hitting windscreen, steering wheel, or dashboard.
Various substance mixtures are known as gas-generating substance mixtures. Thus, for example, sodium azide is used as a gas-supplying principal component; potassium nitrate, as an oxidizing agent, and silicon dioxide, with silicon dioxide chemically binding as slag, the substances sodium and potassium formed as a result of the azide reacting with the nitrate. A major disadvantage of using propellants containing sodium azide is their high toxicity. This requires special measures, for manufacture, transporting, and disposal, among others. A further disadvantage lies in the alkaline reaction of combustion residues, liable to cause a corrosion hazard.
In addition, gas-generating masses are known, which consist of an alkali metal azide and a metal oxide, mostly of iron oxide (cf. DE-OS 24 59 667). However, these known substance mixtures are characterized by a slow combustion speed and poor ignitability. Furthermore, a substance mixture comprising nitrocellulose and nitroglycerin is known from DE-OS 43 17 727. Such propellant mixtures, based on nitrocellulose, are characterized by poor temperature stability; this entails a limited life and makes it impossible to recycle this substance. Moreover, these propellant mixtures contain heavy-metal salts as combustion controllers, which additionally makes disposal difficult. The greatest disadvantage, however, is the enormous quantities of carbon monoxide arising during combustion.
In recent years, sodium-azide-free propellant mixtures have been proposed (U.S. Pat. No. 4,948,439) containing as their principal component organic compounds rich in nitrogen such as tetrazoles or tetrazole derivates or tetrazolates. However, the disadvantage of such nitrogenous organic propellant mixtures is that on combustion considerable quantities of nitrous gases NO.sub.x will be released, in addition to carbon monoxide, so that a combined poisoning hazard cannot be excluded.
Finally, the publication U.S. Pat. No. 3,880,595 reveals substance mixtures based on a nitrogen-free organic compound such as, for example, citric acid. The disadvantage of these propellants is their low thermal stability and their high hygroscopicity as well as their poor processability, in particular, the great difficulties experienced when attempting to compress these substances into tablets or pellets.